Geyser pumping actions may be used for a variety of devices such as solar heating systems wherein solar thermal energy is transferred from a collector panel to a storage medium as sensible heat in a pumped liquid. One such system is shown and described in U.S. Pat. No. 4,478,211 issued Oct. 23, 1984 which is incorporated herein by reference. The storage medium may be placed any reasonable distance below the top of the geyser pump tubes. This is in contrast with thermosiphon systems which achieve liquid flow by the density difference between hotter and colder regions in the liquid and which require the storage medium to be above the riser tubes. Also, a geyser pumped system eliminates the need for controls, valves, sensors and mechanical pumps such as are required by conventionally pumped heat transfer systems. The geyser pumping action starts and stops automatically in response to the potential for transferring heat and the pumping rate is approximately proportional to the heat transfer rate.
By way of illustration the present invention relates to a solar heat transfer system wherein the pumping action is produced by vapor bubble formation in saturated or slightly superheated liquid in the riser tubes (geyser pump tubes) of a solar collector panel. The vapor bubble formation, expansion and buoyancy cause a gravity imbalance which in turn causes forced circulation of the liquid heat transfer medium through the entire system. After the liquid has been cooled by transferring heat to the storage medium and before it is returned to the riser tubes for reheating, it is used to condense the vapor formed in the riser tubes thereby conserving the energy used to produce the pumping vapor and maintaining the pressure in the system at a suitably low value. The condensation occurs directly on a free surface of the cooled liquid.
A geyser pumped system is generally filled with liquid heat transfer fluid except at the top where the riser tubes enter a vapor filled upper manifold which also serves as a vapor/liquid separator. However, when the storage medium (which is lower in the system) is hotter than the upper manifold, the vapor space appears in the heat exchanger. In the latter condition and with proper configuration of the volumes of the upper manifold and the heat exchanger, a vapor block may be formed in the heat exchanger to prevent reverse pumping and consequent loss of heat from the storage medium
One of the problems encountered with geyser pumps relates to the continued, regular generation of vapor bubbles which produce the pumping action. As initially fabricated, the geyser pump system pumps liquid and transfers heat very well for several hours and cycles of operation. However, eventually the regular formation of vapor bubbles in the geyser pump tubes ceases because the natural nucleation sites (where vapor bubbles can readily form) become non-functional. This is a common and widely known phenomenon in nucleate boiling devices and probably occurs because of the depletion of absorbed non-condensable gasses. Without active nucleation sites a vapor bubble cannot form except in the presence of significant superheating. Superheats of 50 to 100 degrees F. above the boiling point are not uncommon and superheats of over 400 degrees F. have been reported. Such high temperatures cause excessive heat loss and reduced efficiency. Also, when the change of state from liquid to vapor occurs in the presence of such superheating, it happens extremely rapidly and a large fraction of the liquid in the tube flashes to vapor. This results in a mixture of vapor and liquid droplets emerging from the top of the riser tubes. Since the function of the geyser pump tubes is to pump liquid, avoidance of excess vapor production through the maintenance of effective bubble formation sites would improve geyser pumping action.
Another problem with geyser pumps relates to the expansion of the vapor bubble in a downward (upstream) direction under some conditions. At high heating rates buoyancy forces are not always adequate to force expansion to occur only in the desired upward (downstream) direction. Reverse expansion dissipates some of the pumping energy, can cause undesirable oscillations and can significantly reduce the liquid pumping rate.
A further problem with geyser pumps relates to possible undesirably high velocities of the liquid emerging from the top of the riser tubes. At high heating rates and with reverse expansion suppressed, the upward expansion of the vapor bubble can be sufficiently rapid as to cause excessive liquid velocities
Therefore it is one object of the present invention to provide a heat transfer system which is self pumping and self regulating by a geyser pump action which is enhanced by incorporation of reliable vapor bubble generation means or liquid flow control means or both.
Another object of the present invention is to provide improved pumping action for a geyser pump tube by isolating, within the tube, a small volume of the working fluid in a partially enclosed volume to provide a continuously active site for the generation of vapor bubbles.
Yet another object of the present invention is to provide enhanced pumping action for a geyser pump tube in which the isolated volume is enclosed in such a manner that good thermal contact is provided with the heated surface of the geyser pump tube.
Still another object of the present invention is to provide enhanced pumping action for a geyser pump tube by including a directional flow restriction in the geyser pump tube to prevent oscillations in the pumping action and to direct the expansion forces of the vapor bubble in the desired direction.
Still another object of the present invention is to enhance pumping action of a geyser pump tube by including a flow constriction downstream of the bubble generator, preferably at the top end of the geyser pump tube. Such a constriction moderates the peak liquid flow by causing the pressure in the vapor bubble to increase with the flow rate, thereby reducing the expansion rate.